Superheater flow baffling

ABSTRACT

Flow baffle structure is provided for a shell and tube vapor generator for directing the flowing vaporizable fluid through the shell in heat exchange relation to the heating fluid flowing through the tubes. The structure comprises a plurality of axially spaced assemblies each containing a number of horizontally disposed, radially extending baffle plates. The plates are sector-shaped and circumferentially spaced from one another to form openings through which the vaporizable fluid is caused to flow. The plates and intermediate openings in adjacent assemblies are alternately disposed so as to direct the fluid along substantially sinuous courses through the affected region of the unit.

[ Mar. 26, 1974 SUPERHEATER FLOW BAFFLING [75] Inventors: John W. Alden, Jr., Karlskrona,

Sweden; Kenneth E. Webster, Chattanooga, Tenn.

[73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn.

[22] Filed: Dec. 13, 1972 [21] Appl. No.: 314,877

3.731.733 5/1973 Trepaud 165/161 Primary Examiner-Kenneth W. Sprague Attorney, Agent, or FirmJohn F. Carney [57] ABSTRACT Flow baffle structure is provided for a shell and tube vapor generator for directing the flowing vaporizable fluid through the shell in heat exchange relation to the heating fluid flowing through the tubes. The structure comprises a plurality of axially spaced assemblies each containing a number of horizontally disposed, radially extending baffle plates. The plates are sector-shaped and circumferentially spaced from one another to form openings through which the vaporizable fluid is caused to flow. The plates and intermediate openings in adjacent assemblies are alternately disposed so as to direct the fluid along substantially sinuous courses through the affected region of the unit.

12 Claims, 8 Drawing Figures PATENIEU MR2 5 I974 SHEU 2 OF 2 FIG. 8

FIG.

SUPERHEATER FLOW BAFFLING BACKGROUND OF THE DISCLOSURE The invention relates to shell and tube vapor generators in which a vaporizable liquid is evaporated and superheated by the indirect transfer of heat from a high temperature fluid. Vapor generators of this type normally comprise a tube bundle through which the heating fluid is conducted and an upright shell enclosing the tube bundle and within which the vaporizable liquid is circulated in heat transfer relation to the tubes of the tube bundle.

In the operation of the vapor generators of this type the effectiveness of heat transfer between the two me- ,dia, as represented by the heat transfer coefficient, is markedly reduced in the high temperature regions of the unit. These regions include the superheat region and the liquid deficient" region, the latter being defined as that region within which film boiling, as contrasted with nucleate boiling, occurs. It is known that the heat transfer coefficient in these regions can be improved by directing the vaporizable fluid transversely of the tubes rather than parallel to their axes. To achieve maximum heat transfer conditions in a shell and tube vapor generator, therefore, the unit can be designed for directing the vaporizable fluid in pure cross flow relation to the tubes. This type of flow is defined as that wherein the direction of fluid flow across the tubes is substantially perpendicular to the tube axes. Pure cross flow can be obtained by the use of axially spaced, horizontally disposed, annular baffles, commonly referred to as disc-donut baffles, along the flow path of the vaporizable liquid. Such practice is effective only at the expense of a loss of kinetic energy in the. vaporizable fluid, which is manifested as fluid pressure loss. This loss results from inpingement of the fluid upon the tube surfaces; the repeated flow reversals imparted to the fluid in its coursing through the affected region of the vapor generator; and the repeated accelerations and decelerations undergone by the fluid in flowing radially inwardly and outwardly along the paths defined by the disc-donut baffles within the boundaries of the cylindrical containment chamber.

In most applications, therefore, obtaining maximum heat transfer by directing the vaporizable fluid in pure cross flow relation to the heat exchange tubes is, from a practical standpoint, unfeasible due to the high degree of pressure drop attendant therewith. It is further undesirable because of the fact that containment shells of increased diameter must be employed since, to achieve pure cross flow, the flow reversals from pass to pass must occur beyond the boundaries of the tube bundle. The space that must be provided to accommodate these flow reversals is otherwise wasted since no heat transfer occurs therewithin. This, of course, adds considerably to the cost of the unit.

Contemporary shell and tube vapor generator design seeks to obtain a compromise between maximum heat transfer conditions and minimum pressure drop conditions. It has been the practice, therefore, to employ baffle arrangements in the higher fluid temperature regions of the unit similar to the above-described discdonut arrangement between wherein flow reversals occur completely within the boundaries of the tube bundle. In these units the vaporizable fluid flows in oblique" relation to the tubes having directional components both parallel to the axes of the tubes and perpendicular thereto. Use of this form of baffle arrangement produces a moderately high heat transfer coefficient within tolerable pressure drop conditions. Its performance is limited, however, by the fact that the flow baffles are customarily mounted upon the tube support structures in these units and the axial distance between these structures is fixed by tube vibration considerations. Thus, it is impossible to alter the position of the baffles within close limits when it is desired to vary fluid flows therebetween in order to make fine adjustments in the heat transfer and pressure .loss conditions in the unit.

It is to the solution of these problems, therefore, that the present invention is directed.

SUMMARY OF THE INVENTION According to the present invention a flow baffle arrangement is provided for use in a shell and tube vapor generator, especially in the superheat and liquid deficient regions thereof, for directing the vaporizable fluid in heat transfer relation to the tubes in a manner as will avoid the above-cited problems and achieve optimum operating conditions. The flow baffle arrangement described herein comprises a plurality of axially spaced assemblies, each including a plurality of circumferentially spaced baffle plates of generally sector shape. The positions of the baffle plates and the intermediate spaces therebetween are reversed in adjacent assemblies to direct the fluid along sinous courses through those regions of the vapor generator within sinuous the apparatus is located.

Vapor generators employing the described baffle arrangement are characterized by high heat transfer in relation to the amount of pressure drop experienced by the vaporizable fluid attendant therewith. Moreover, use of this baffle arrangement permits the vapor heating conditions in shell and tube vapor generators to be optimized in that the baffles, although mounted on tube supported structures maintained at fixed axial spacing, can be designed to vary the pressure drop and heat transfer conditions within the unit.

For a better understanding of the invention, its operating advantages and the specific objects obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred embodiment of the invention.

DESCRIPTION OF THE DRAWINGS FIG. I is an elevational view of a shell and tube vapor generator incorporating the present invention;

FIG. 2 is a plan sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a plan sectional view taken along line 303 of FIG. 1;

FIG. 4 is a partial plan sectional view taken along line 4-4 of FIG. 1;

FIG. 5 is an enlarged elevational view of a typical tube support structure employed in the vapor generator of FIG. 1;

FIG. 6 is an enlarged elevational view similar to FIG. 5 but .illustrating a baffle plate according to the invention attached to the tube support structure;

FIG. 7 is a plan view of a typical sector baffle of the present invention; and

FIG. 8 is a schematic perspective representation illustrating the flow baffle arrangement according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The vapor generator 10 comprises a vertically elongated cylindrical shell 12 having its upper and lower ends closed by dome-shaped closure heads 14 and 16 respectively to define a substantially closed vessel. A pair of axially spaced upper and lower tube sheets 18 and 20 are provided adjacent the respective ends of the shell. The tube sheets extend transversely of the shell axis and have their outer peripheral edges attached to the shell wall to divide the vessel into three axially spaced chambers. The upper and lower chambers, 22 and 24, are termed the heating fluid inlet and outlet chambers respectively and the intermediate chamber is referred to as the evaporation chamber 26. A plurality of straight, thin walled tubes 28 are disposed in a bundle and extend through the evaporation chamber 26' from end-to-end thereof in parallel relation to the shell axis. The ends of the tubes 28 extend through, and are weldedly secured to, the respective tube sheets 18 and 20 to communicate with the heating fluid inlet and outlet chambers 22 and 24 for circulating a high temperature fluid from a source (not shown) in indirect heat transfer relation with vaporizable fluid supplied to the chamber 26.

The herein described vapor generator 10 is provided with an axially disposed conduit 30 of enlarged diameter that extends through the evaporation chamber 26. The opposite ends of the conduit 30 are attached to the tube sheets 18 and 20 about central openings 32 and 34 provided therein to accommodate passage of the heating fluid. Opening 34 in the lower tube sheet 20 connects with heating fluid inlet nozzle 36 by means of a cylindrical connector 38 whereby heating fluid is delivered from the source through the conduit 30 to the heating fluid inlet chamber 22 located at the top of the shell. From the chamber 22 the heating fluid is caused to pass downwardly through the tubes 28 discharging into the outlet chamber 24. Heating fluid outlet nozzle 40 communicates with the outlet chamber 24 and serves to return the spent heating fluid to its source for reheating.

The evaporation chamber 26 in the vapor generator 10 contains partition means in the form of axially spaced cylindrical baffles 44 and 45 that surround the tube bundle in concentric relation to the shell wall. The upper baffle 44 cooperates with the shell wall to define an annular passage 52, the lower end of which is closed by a horizontally disposed, annular plate whose peripheral edges are' secured by welding to the lower end of the baffle 44 and to the shell wall. The lower baffle 45 similarly cooperates with the shell to define an annular downcomer passage 54, the upper end of which is closed by annular closure plate similar to the plate 50. The upper and lower end edges of the respective baffles 44 and 45 are spaced from the adjacent tube sheets 18 and 20 respectively to establish fluid communication between the evaporation chamber 26 and the respective passages 52 and 54.

A feedwater inlet nozzle 56 penetrates the wall of the shell 12 and serves to supply vaporizable fluid in a liquid state to the vapor generator 10. As shown, the feedwater inlet nozzle 56 communicates with the downcomer passage 54 within which the liquid supplied thereto is caused to flow downwardly and thence into the evaporation chamber 26. Within the evaporation chamber the liquid passes upwardly in counterflow, indirect, heat transfer relation with the heating fluid flowing through the tubes 28.

A vapor discharge nozzle 57 communicates with passage 52 and serves to conduct superheated vapor produced in the vapor generator 10 to a point of use.

Tube support structures, indicated generally as 58, are disposed at vertically spaced intervals trliough the height of the tube bundle. These structures serve to provide lateral support for the tubes 28 thereby to stiffen them throughout their length and to reduce the potential damaging affects of vibration to which the tubes would otherwise be subjected. The tube support structures 58 employed in the described vapor generator are of the egg crate type and comprise relatively thin tube spacer bars 60 that extend through the tube bundle along spaces between adjacent tube rows. As is well known, the spacer bars 60 are arranged in two groups with the bars in each group being disposed in mutually parallel relation. The respective groups of bars are disposed in mutually intersecting relation thus to form a grid-like structure through which the tubes 28 of the tube bundle extend. The bars 60 may be connected at their intersections by interengaging slots (not shown) to secure the members together. The ends of the bars 60 that define the inner and outer peripheral boundaries of the tube support structures 58 are connected by concentric pairs of connector rings, indicated as 64 and 66 respectively. As shown in FIG. 5, the rings in each pair are weldedly attached to the upper and lower edges of the bars 60 respectively to form an integral structure. The structures 58 are attached in position to the adjacent surface to the conduit 30 and to a plurality of upstanding structural columns or T-bars 61. The structural columns 61 are, themselves, joined along their webs to the inner surface of the shell by welding the pairs of rings 64 and 66 to the surfaces of the respective members.

In the lower regionof the evaporation chamber 26 flow baffles are provided for directing the incoming liquid along annular, sinuous courses in heat exchange relation to the tubes 28. The baffle structure disclosed herein is of the conventional disc-donut design and comprises a number of horizontally disposed axially spaced annular plates 70 and 72 that are mounted upon the tube support structures 58 in this region of the passage. As indicated in FIG. 2, plates 70 are attached about their outer peripheral edges to the cylindrical baffle 45 and have their inner peripheral edges terminating short of the central conduit 30 to form an annular space therebetween for the flow of fluid. Plates 72 (only one of which is shown in FIG. 1) on the other hand, are attached about their inner periphery to the conduits 30 with their outer edges being spaced from the inner surface of the cylindrical baffle 45. The plates 70 and 72 thus cooperate to direct the liquid supplied to the riser passage 48 along an annular, substantially sinusoidally course through this region of the passage. By means of this baffling structure the liquid is directed radially back and forth across the surface of the tubes 28 in oblique heat transfer relation therewith.

According to the present invention, flow baffling of particular configuration is disposed in the upper portion of the evaporation chamber 26 to direct the flow of vaporizable fluid across the tubes 28. For the purpose of this description the upper region of evaporation chamber 26 includes the superheat region, indicated generally as 74 and the liquid deficient region 76, i.e., that portion of the evaporation chamber wherein the vaporizable fluid is in a two phase condition but predominantly vapor and wherein film boiling occurs. For present purposes the liquid deficient" region includes that portion of the evaporation chamber 26 within which the vaporizable fluid is above 80 percent quality although it should be understood that fluid of higher or lower quality is also contemplated. ln FIG. 1 the lower limit of this region is indicated by the line 78. Its upper limit, i.e., where the fluid is 100 percent vapor, is indicated by line 80.

The flow baffling structure employed in this region of the vapor generator comprises a plurality of vertically spaced baffle assemblies indicated generally as 82. Each baffle assembly 82 includes a plurality of horizontally disposed, radially extending, circumferentially spaced baffle plates 84. The plates 84 are flat, fluid impervious members containing a plurality of through openings 86 to accommodate passage of the tubes 28. The plates 84 have a peripheral configuration generally similar to that of the sector of the annulous between the central conduit 30 and the baffle 44. As shown in HO. 6 the plates 84 are attached at their radially inner and outer edges 88 and 90 respectively to the lower rings and their respective connector ring pairs 64 and 66 forming part of the associated tube support structure 58.

In the preferred form the plates 84 are circumferentially spaced from one another such that the spaces 92 therebetween are of substantially the same peripheral shape and size and occupy substantially the same transverse area as the plates. The assemblies 82 are each identically formed, but alternate assemblies, as shown in FIGS. 2 and 8, have the position of the plates 84 and intermediate spaces 92 reversed. Thus the vaporizable fluid transversing this region of the riser passage 48 is caused to flow vertically along a plurality of circumferentially spaced, generally sinuous courses, as indicated by the arrows 94 in FIG. 8. The fluid flows in this manner through the liquid deficient and superheat regions, 76 and 74 respectively, to the top of the cylindrical baffle 44 from whence it passes into the annular discharge passage 52 as superheated vapor and is subsequently discharged through nozzle 55.

As is evident from examination of FIG. 8 of the drawing the vaporizable fluid, in flowing through that region of the evaporation chamber 26 in which the flow baffle assemblies 82 are located, is directed in oblique relation to the tubes 28 thus achieving an effective transfer of heat between the two fluid media. At the same time, pressure losses in the vaporizable fluid are maintained at a reduced level since the fluid in passing through the planes of the baffle assemblies 82 flows substantially parallel to the axes of tubes 28. The result is that a more effective heat transfer is obtained for a given amount of pressure loss when compared with similar vapor generators employing flow baffling structures of heretofore known design.

Through use of the present invention designers of shell and tube vapor generators can optimize the selection of heat exchange tube surfaces regardless of tube support structure spacing since the flow areas and, therefore, pressure loss through the flow baffle assem bly is independent of the tube support structure spacing. Alterations can thereby be made in flow velocities by simply increasing or decreasing the number and/or size of the sector shaped baffle plates employed in each assembly.

Additionally, use of the flow baffle arrangement of this present invention overcomes the problem of excessive fluid velocities in the upper portion of the superheat region of the evaporation chamber where, with constant flow areas, fluid velocities have a natural tendency to increase due to the increase in specific volume of the fluid produced by increase fluid temperature. This tendency can be offset through use of the present invention by simply increasing the flow area through the uppermost flow baffle assemblies by adjusting the size and/or number of baffle plates employed therein.

Still another advantageous feature of the present invention is that vaporizable fluid flow patterns through the region of the vapor generator in which the flow baffle assemblies are located are rendered more predictable thus facilitating selection of the amount of tubular heating surface required in the unit. Because the fluid flow areas in the baffle assembly are substantially uniform in the axial and circumferential directions the flow velocity of the vaporizable fluid with respect to the tube surface disposed intermediate the plates is essentially constant and, therefore, can be accurately predicted. This is a significant improvement over conventional flow baffling in which fluid is directed radially through the tube bundle thereby experiencing constant changes in flow velocity, decelerating as it flows outwardly from the shell axis and accelerating as it flows inwardly thereto. Thus because fluid flow patterns can be accurately predicted the amount of tube surface required in the unit to produce the desired output can be accurately calculated.

For a better understanding of the invention, its operating advantages and the specific objects obtained by its use, reference should be made to the accompanying drawings and description which relate to various embodiments of the invention.

What is claimed is:

1. A forced flow heat exchanger for the generation of vapor by the indirect transfer of heat between two fluid media, comprising:

a. an elongated cylindrical shell;

b. partition means forming an annular riser passage within said shell;

c. a plurality of heat exchange tubes within said riser passage;

d. means for circulating heating fluid through said tubes;

e. means for supplying vaporizable fluid in a liquid state to one end of said riser passage;

f. means for removing generated vapor from said shell; and

g. means for directing the flow of said vaporizable fluid in heat exchange relation to said tubes including a flow baffle assembly traversing said riser passage at each of a plurality of axially spaced locations along said riser passage, each of said assemblies comprising a plurality of horizontally disposed baffle plates circumferentially spaced about said riser passage, each of said baffle plates and the intermediate spaces therebetween being shaped generally as a sector of said riser passage and means in said plates to acommodate passage of said tubes.

2. Apparatus as recited in claim 1 wherein said baffle plates and said intermediate spaces are disposed in alternate positions in adjacent assemblies.

3. Apparatus as recited in claim 2 wherein said flow baffle assemblies are located in the superheat region of said riser passage.

4. Apparatus as recited in claim 2 wherein said flow baffle assemblies are located in the liquid deficient region of said riser passage.

5. Apparatus as recited in claim 2 wherein said baffle plates and said intermediate spaces therebetween are of substantially peripheral shape.

6. Apparatus as recited in claim 2 including tube support structures disposed at regular, axially spaced intervals throughout the length of said riser passage, said flow baffle assemblies comprising selected ones of said tube support structures, and means for attaching said baffle plates to said selected tube support structures.

7. A forced flow heat exchanger for the generator of vapor by the indirect transfer of heat between two fluid media, comprising:

a. a vertically elongated cylindrical shell;

b. a pair of axially spaced tube sheets extending transversely of the axis of said shell and dividing the same into axially spaced heating fluid manifold chambers and an intermediate evaporation chamber;

c. an axially disposed conduit extending between, and having the opposite ends thereof attaching, said tube sheets;

d. generally cylindrical baffle means concentrically spaced from the wall of said shell and said conduit dividing said evaporation chamber into concentric annular downcomer and riser passages, the end of said cylindrical baffle being axially spaced from said lower tube sheet placing said downcomer and riser passages in fluid communication;

. a plurality of substantially straight heat exchange tubes extending between said tube sheets through said riser passage, said tubes having the opposite ends thereof in fluid communication with said heating fluid manifold chambers;

f. means for circulating heating fluid through said manifold chambers and said tubes;

g. means for supplying vaporizable fluid in a liquid state to said downcomer and riser passages;

h. means for removing generated vapor from said shell; and

i. means for directing the flow of said vaporizable fluid in heat exchange relation to said tubes including a flow baffle assembly traversing said riser passage at each of a plurality of axially spaced locations along said riser passage, each of said assemblies comprising a plurality of horizontally disposed baffle plates circumferentially spaced about said riser passage, each of said baffle plates and the intermediate spaces therebetween being shaped generally as a sector of said riser passage, and means in said plates to accommodate passage of said tubes.

8. Apparatus as recited in claim 7 wherein the baffle plates and the intermediate spaces therebetween are disposed in alternate positions in adjacent assemblies.

9. Apparatus as recited in claim 8 wherein said flow baffle assemblies are disposed throughout the superheat region of said riser passage.

10. Apparatus as recited in claim 9 wherein said flow baffle assemblies are disposed throughout the liquid deflcient region of said riser passage.

11. Apparatus as recited in claim 8 including tube support structures disposed at regular, axially spaced intervals throughout the height of said riser passage, said flow baffle assemblies comprising selected ones of said tube support structures, and means for attaching said baffle plates to said selected tube support structures.

12. Apparatus as recited in claim 8 wherein said baffle plates and said intermediate spaces therebetween in the respective flow baffle assemblies are of substantially identical peripheral shape. 

1. A forced flow heat exchanger for the generation of vapor by the indirect transfer of heat between two fluid media, comprising: a. an elongated cylindrical shell; b. partition means forming an annular riser passage within said shell; c. a plurality of heat exchange tubes within said riser passage; d. means for circulating heating fluid through said tubes; e. means for supplying vaporizable fluid in a liquid state to one end of said riser passage; f. means for removing generated vapor from said shell; and g. means for directing the flow of said vaporizable fluid in heat exchange relation to said tubes including a flow baffle assembly traversing said riser passage at each of a plurality of axially spaced locations along said riser passage, each of said assemblies comprising a plurality of horizontally disposed baffle plates circumferentially spaced about said riser passage, each of said baffle plates and the intermediate spaces therebetween being shaped generally as a sector of said riser passage and means in said plates to acommodate passage of said tubes.
 2. Apparatus as recited in claim 1 wherein said baffle plates and said intermediate spaces are disposed in alternate positions in adjacent assemblies.
 3. Apparatus as recited in claim 2 wherein said flow baffle assemblies are located in the superheat region of said riser passage.
 4. Apparatus as recited in claim 2 wherein said flow baffle assemblies are located in the liquid deficient region of said riser passage.
 5. Apparatus as recited in claim 2 wherein said baffle plates and said intermediate spaces therebetween are of substantially peripheral shape.
 6. Apparatus as recited in claim 2 including tube support structures disposed at regular, axially spaced intervals throughout the length of said riser passage, said flow baffle assemblies comprising selected ones of said tube support structures, and means for attaching said baffle plates to said selected tube support structures.
 7. A forced flow heat exchanger for the generator of vapor by the indirect transfer of heat between two fluid media, comprising: a. a vertically elongated cylindrical shell; b. a pair of axially spaced tube sheets extending transversely of the axis of said shell and dividing the same into axially spaced heating fluid manifold chambers and an intermediate evaporation chamber; c. an axially disposed conduit extending between, and having the opposite ends thereof attaching, said tube sheets; d. generally cylindrical baffle means concentrically spaced from the wall of said shell and said conduit dividing said evaporation chamber into concentric annular downcomer and riser passages, the end of said cylindrical baffle being axially spaced from said lower tube sheet placing said downcomer and riser passages in fluid communication; e. a plurality of substantially straight heat exchange tubes extending between said tube sheets through said riser passage, said tubes having the opposite ends thereof in fluid communication with said heating fluid manifold chambers; f. means for circulating heating fluid through said manifold chambers and said tubes; g. means for supplying vaporizable fluid in a liquid state to said downcomer and riser passages; h. means for removing generated vapor from said shell; and i. means for directing the flow of said vaporizable fluid in heat exchange relation to said tubes including a flow baffle assembly traversing said riser passage at each of a plurality of axially sPaced locations along said riser passage, each of said assemblies comprising a plurality of horizontally disposed baffle plates circumferentially spaced about said riser passage, each of said baffle plates and the intermediate spaces therebetween being shaped generally as a sector of said riser passage, and means in said plates to accommodate passage of said tubes.
 8. Apparatus as recited in claim 7 wherein the baffle plates and the intermediate spaces therebetween are disposed in alternate positions in adjacent assemblies.
 9. Apparatus as recited in claim 8 wherein said flow baffle assemblies are disposed throughout the superheat region of said riser passage.
 10. Apparatus as recited in claim 9 wherein said flow baffle assemblies are disposed throughout the liquid deficient region of said riser passage.
 11. Apparatus as recited in claim 8 including tube support structures disposed at regular, axially spaced intervals throughout the height of said riser passage, said flow baffle assemblies comprising selected ones of said tube support structures, and means for attaching said baffle plates to said selected tube support structures.
 12. Apparatus as recited in claim 8 wherein said baffle plates and said intermediate spaces therebetween in the respective flow baffle assemblies are of substantially identical peripheral shape. 